Referring to FIG. 1, a phase-frequency detector 10 and a charge pump 12 for a conventional phase lock loop are shown. The phase-frequency detector 10 comprises two flip-flops 14 and 16, a logic gate 18, and a delay circuit 20. The charge pump 12 comprises a current source 22, a current source 24, a switch, SW_PU, and a switch SW_PD. The charge pump 12 may have an interface 26 to present an output signal (i.e., IOUT).
The flip-flop 14 presents a signal (i.e., PUMP_UP) in response to a rising edge of a reference signal (i.e., REF). The other flip-flop 16 presents a signal (i.e., PUMP_DOWN) in response to an input signal (i.e., IN). The logic gate 18 performs a logical AND function on the signal PUMP_UP and the signal PUMP_DOWN to present a reset signal (i.e., RESET). The delay line 20 delays the signal RESET back to the flip-flops 14 and 16. The signal PUMP_UP is also presented to the switch SW_PU in the charge pump 12. The signal PUMP_DOWN is presented to the switch SW_PD in the charge pump 12. While the signal PUMP_UP is in an active state, the switch SW_PU closes causing a current signal (i.e., 11) to flow from the current source 22 to the signal IOUT. While the signal PUMP_DOWN is in active state, the switch SW_PD closes causing a current signal (i.e., 12) to flow from the current source 24 to the signal IOUT.
Referring to FIG. 2, a timing diagram of the signals shown in FIG. 1 is provided. The timing diagram shows a scenario where the signal REF transitions low to high before the signal IN. In a scenario where the signal IN transitions low to high before the signal REF then the signal PUMP_DOWN will become active before the signal PUMP_UP thus causing the signal IOUT to have a negative value.
A rising edge 30 in the signal REF will cause a transition 32 in the signal PUMP_UP from an inactive state to an active state. The signal PUMP_UP transition 32 to the active state causes a transition 34 in the signal I1 from a non-flowing state to a flowing state. The signal I1 is added to the signal IOUT causing a transition 36 in the signal IOUT to a positive non-zero value.
A rising edge 38 in the signal IN causes a transition 40 in the signal PUMP_DOWN from the inactive state to the active state. The signal PUMP_DOWN transition 40 results in a transition 42 in the signal I2 from the non-flowing state to the flowing state. The signal I2 is added to the signal IOUT causing a transition 44 in the signal IOUT back to a zero value. The signal I1 and the signal I2 are conventionally designed to be identically opposite currents. As a result, while both the signal I1 and signal I2 are flowing, the signal IOUT has the zero value.
The combination of the signal PUMP_UP and the signal PUMP_DOWN in the active state may cause the signal RESET to become active at an output of the logic gate 18. The delay circuit 20 will delay the signal RESET for a delay period (e.g., MARGIN). At the end of the delay period MARGIN, a transition 46 takes place in the signal RESET from the inactive state to the active state. The signal RESET transition 46 causes a transition 48 in the signal PUMP_UP and a transition 50 in the signal PUMP_DOWN to the inactive state. The signal PUMP_UP in the inactive state causes a transition 52 in the signal I1 to the non-flowing state. The signal PUMP_DOWN in the inactive state causes a transition 54 in the signal I2 to the non-flowing state. The signal PUMP_UP and/or the signal PUMP_DOWN in the inactive state causes a transition 56 in the signal RESET to the inactive state.
A duration of the signal PUMP_UP minus a duration of the signal PUMP_DOWN equals a difference in arrival times of edges in the signal REF and the signal IN. The difference in arrival times determines a duration of the signal IOUT at the non-zero value. The delay period MARGIN determines an overlap between the signal I1 and the signal I2.
The purpose of the delay circuit 20 or the delay period MARGIN is to make sure that small phase differences between the signal REF and the signal IN cause the signal I1 and/or signal I2 to be switched in the charge pump 12. If the phase difference between the signal REF and the signal IN is at or near zero, then the switch SW_PU and the switch SW_PD will close approximately simultaneously. Once both the switch SW_PU and the switch SW_PD are closed, the current presented by the upper current source 22 is sinked by the lower current source 24. Current source 24 sinking the current source 22 establishes the signal IOUT with the zero value.
Without the delay circuit 20 in the phase-frequency detector 10, a risk is incurred that the signal PUMP_UP and the signal PUMP_DOWN can be too small to cause the switch SW_PU and/or the switch SW_PD to close. Consequently, the phase-frequency detector 10 and the charge pump 12 would not react on very small phase differences between the signal REF and the signal IN.
A problem with the conventional technology is that in order to ensure an overlap in the signal I1 and the signal I2 in all situations, the delay circuit 20 must delay the signal RESET for a worst-case situation. The worst-case situation is commonly influenced by process variations, temperature variations, and power supply voltage variations. The worst-case delay may cause several drawbacks. For example, a crowbar current will flow through the charge pump 12 during the delay period MARGIN. Since the delay period MARGIN is longer than required in most situations, an excessive amount of power is consumed.
Another undesirable situation occurs when the signal I1 and the signal I2 are not evenly matched. When the signal I1 and the signal I2 are not matched, the signal IOUT has a non-zero value during the delay period MARGIN. As a result, the charge pump 12 may present the signal IOUT with a small current that will cause a phase offset in the phase lock loop.
A third situation occurs when the delay period MARGIN is too short. A very short delay period MARGIN commonly results in no overlap between the signal I1 and the signal I2. The non-overlap results in a deadband in the response of the phase lock loop. Slight changes to the phase difference between the signal REF and the signal IN will not result in a change to the signal IOUT.